1. Field of the Invention
The present invention relates to a transfer case for transferring a drive torque to a front axle in an all-wheel driving system for a rear wheel driving vehicle. More particularly, the present invention relates to a range shift mechanism in the form of a planetary gear assembly system shifting the speed between a high range and a low range in a transfer case.
2. Description of the Related Art
A conventional transfer case, for example, the transfer case disclosed in U.S. Pat. Nos. 7,399,251 and 6,616,571, is a device of a 4-wheel or all-wheel driving system for a rear wheel driving vehicle. The transfer case receives a driving force from a transmission and transfers the driving force to a front axle and a rear axle. The transfer case supports two types of range shift mechanisms, that is, a mode shift mechanism and a range shift mechanism.
FIG. 1 is a schematic diagram illustrating a driveline of a general 4-wheel driving vehicle including a transfer case 100.
In a rear wheel driving vehicle, the driving force of an engine 1 is transferred to a rear wheel R through a transmission 2, a rear wheel driving shaft 3 and a rear axle assembly 5. In order to establish a 4-wheel or all-wheel system, the transfer case 100 is installed between the transmission 2 and the rear wheel driving shaft 3. In addition, the transfer case 100 distributes the driving force to a front wheel F through the front axle assembly 4.
The transfer case 100 includes a mode shift mechanism 110 and a range shift mechanism 120. The mode shift mechanism 110 of the transfer case 100 provides shifting between the 2-wheel driving mode and a 4-wheel driving mode, and the range shift mechanism 120 provides shifting between a high range mode and a low range mode.
The high range mode is selected to ensure safe running by increasing a ground contact force through uniform power distribution of 25% for each of front and rear wheels to prevent slips or tire damages due to excessive power of 50% transferred to the rear wheels on an unpaved road or a road in a bad condition. During normally running, even when the road condition is bad due to snow or rain, highly efficient driving can be delivered. In the high range mode, the driving speed is constant relative to a transmission speed in a two-wheel drive mode (average gear ratio of 1:1).
The low range mode is selected to ensure maximum traction force and high speed running. In the low range mode, high driving performance is demonstrated in running on the road of a steep slope or in pulling a vehicle. In particular, the low range mode allows excellent escape performance on an unpaved road in a severely adverse condition (a muddy road, a creviced road, a road with a side slope, a road with a steep slope, etc.). In the low range mode, the driving speed has a high reduction ratio relative to a transmission speed in a two-wheel drive mode (average gear ratio of around 1:2).
FIG. 2 is a cross-sectional view of a general range shift mechanism 120 including the transfer case 100 shown in FIG. 1.
The range shift mechanism 120 includes a planetary gear assembly system 130, and the planetary gear assembly system 130 includes a sun gear 131, a pinion gear 132, a ring gear 133 and a carrier 135.
In the general range shift mechanism 120, the ring gear 133 is fixed to the housing 121, and the sun gear 131 and the pinion gear 132 rotate within the ring gear 133 that is fixed all the time for power transfer. Here, undefined reference numeral “141” denotes a sleeve connected to a shift lever.
The mode shift mechanism allows a driver to shift the drive mode from a “2-wheel drive mode” to “4-wheel drive mode” while the range shift mechanism 120 allows shifting between a “high range mode” and a “low range mode”.
The range shift mechanism 120 typically includes a planetary gear assembly system 130. In the high range mode, the sun gear 131 is connected to an output shaft OP, and speed reduction occurs in the planetary gear assembly system 130. The low range mode is employed only under a low-speed condition or a heavily loaded condition, for example, such a case of pulling of a heavy load or driving on an unpaved road. The percentage of using the low range mode is 1% or less.
In the general range shift mechanism 120, the ring gear 133 is fixed to the housing 121, and continuous gear rotation occurs all the time between the sun gear 131, the pinion gear 132 and the ring gear 133. In the high range mode used by 99% or greater in the general range shift mechanism 120, speed reduction by the planetary gear assembly system 130 is not required. In the high range mode in which the sun gear 131 is directly connected to the output shaft OP, the pinion gear 132 rotates between the sun gear 131 and the ring gear 133. Such gear rotation causes a loss in the power and gear noises.